This invention relates to a heat conductive sheet and a production method thereof. More specifically, this invention relates to a heat conductive sheet, which is useful as a heat transfer medium of electronic components and so forth, and a production method thereof
Dissipation of heat from heat generating members has become a problem in various fields. In a variety of devices, such as electronic devices, personal computers, and so forth, in particular, removal of heat from heat generating electronic components and other components (hereinafter generically referred to as the xe2x80x9cheat generating componentsxe2x80x9d) incorporated in these devices has become a serious problem. The probability of erroneous operations of various heat generating components is likely to increase exponentially as the temperature of the components rises. Because these heat generating components have become smaller and smaller in size, and the processing speed has become higher and higher in recent years, the requirement for heat radiation performance has become all the more important.
Various heat radiation members such as a heat sink, a heat radiation fin, a metal heat radiation plate, etc. have been incorporated with heat generating components in order to dissipate the heat generated from, and built up in, the heat generating components. Also, various heat transfer sheets have been used as a heat transfer spacer and as a heat transfer medium between the heat generating components and the heat radiation member. A heat transfer spacer exhibiting high heat conductivity of at least 2.0 W/mxc2x7K and sufficiently reduced heat resistance in the packages has become necessary, particularly in recent years, in order to cope with remarkable exothermy resulting from a higher output operation of electronic devices.
Most of the conventional heat conductive sheets comprise the blend of a silicone rubber and a filler for improving heat conductivity. Examples of this filler are alumina, silica (quartz), boron nitride, magnesium oxide, and so forth. As a concrete example, Japanese Unexamined Patent Publication (Kokai) No. 56-837 describes a heat radiation sheet comprising an inorganic filler and a synthetic rubber such as a silicone rubber as the principal components, wherein the inorganic filler comprises two components of (A) boron nitride and (B) alumina, silica, magnesia, zinc white and mica. Japanese Unexamined Patent Publication (Kokai) No. 7-111300 describes an insulating, heat radiation sheet formed by causing boron nitride powder having a thickness of at least 1 xcexcm to be co-present with a silicone rubber. Japanese Unexamined Patent Publication (Kokai) No. 7-157664 describes a heat conductive silicone rubber sheet that contains at least boron nitride and a ceramic material having the same crystal structure as that of boron nitride or a basic metal oxide in a silicone rubber, and is applied to a woven fabric. Furthermore, Japanese Unexamined Patent Publication Kokai) No. 10-204295 discloses a heat conductive silicone rubber composition useful for forming a sheet, which composition contains (A) a specific organopolysiloxane, (B) boron nitride powder, (C) a fluorine-modified silicone surfactant and (D) a curing agent.
Though these heat conductive silicone rubber sheets exhibit high heat conductivity, they involve several problems yet to be solved. For example, silicone rubber itself is expensive, and its cost is reflected in the cost of the heat radiation sheet. Because the sheet is fabricated by using silicone rubber having a low curing rate, the production process of the sheet is time-consuming Because large amounts of fillers are added in order to improve heat conductivity, a working machine is likely to be worn out with the increase of the viscosity. The production process of such a sheet is complicated, and the production apparatus includes an air heating furnace, a press machine, etc. and becomes large in scale.
The sheet itself of the conventional silicone rubber sheet is hard. Therefore, if the heat generation component or the heat radiation member has a specific shape, such as ruggedness or curvature, the sheet cannot follow such a shape, and the heat resistance increases due to the resulting gaps. If the rubber sheet is pushed strongly in order to eliminate such gaps, delicate electronic components are pushed excessively, and functional troubles are likely to occur.
Attempts have been made in recent years to make the silicone rubber softer so that the rubber sheet can acquire high adhesion capable of following the shapes of the components having complicated shapes. For example, Japanese Unexamined Patent Publication (Kokai) No. 10-189838 discloses a heat conductive gel useful for a heat radiation sheet, which gel is prepared by adding a silicone oil and a heat conductive filler such as boron nitride, silicon nitride, aluminum nitride, magnesium oxide, or the like, by using a condensation type gel such as a condensation curing type liquid silicone gel as a binder, and which is cured to the gel at a normal temperature. Though this heat radiation sheet can obtain high adhesion, however, its heat conductivity remains about 0.8 to 1.1 W/m*K. Therefore, the heat conductivity must be further improved in order to satisfy the recent requirement. If the packing ratio of the filler is increased to obtain higher heat conductivity, plasticity of the gel composition drops and the forming property gets deteriorated. Furthermore, the strength of the heat radiation sheet obtained after curing drops too. Even if the combination of the two kinds of inorganic fillers (A) and (B) described in the aforementioned Japanese Unexamined Patent Publication (Kokai) No. 56-837 is added to the silicone gel, the highest packing ratio that allows sheet forming is at most 45%, and a heat conductive sheet satisfying both requirements for high adhesion and high heat conductivity cannot be acquired.
In addition, another problem is caused if the silicone rubber or other heat conductive sheets are made to be softer. In other words, the heat conductive sheet is offered generally under the state where its tacky surface is covered with a release liner (release paper), and the release liner is peeled immediately before using the sheet. As the sheet thickness becomes smaller to satisfy the requirement for higher heat radiation performance, the heat conductive sheet is likely to get elongated when it is peeled from the release liner, and, when the liner is peeled after bonding, bonding of the sheet in a desired shape becomes difficult
As a means for solving the problem of elongation of the heat conductive sheet it has been customary to use the heat conductive sheet under the state where it is supported by a support such as a plastic film, a metal foil, or the like. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-291226 describes a heat radiation sheet having the feature in that a cured product of a silicone resin composition containing a heat conductive material is applied to one, or both, of the surfaces of a metal foil (foil of aluminum, copper, silver, etc.) having preferably a thickness of 0.01 to 0.05 mm. Japanese Unexamined Patent Publication (Kokai) No. 9-17923 describes a heat conductive sheet characterized by including a silicone gel layer on both surfaces of a support of an aluminum foil, or the like, having preferably a thickness of 0.025 to 0.10 mm. The metal foil used as the support is excellent in heat conductivity. However, because the thickness of the support is 0.02 mm or more, the foil lacks flexibility. When the support is the outermost layer and is in direct contact with the surface of the heat generating component or the surface of the heat radiation member, however, the foil fails to sufficiently follow the surface shape, and desired heat radiation performance cannot be obtained.
Japanese Unexamined Patent Publication (Kokai) No. 8-174765 discloses a heat-resistant heat conductive silicone rubber composite sheet obtained by curing a silicone rubber composition consisting of organopolysiloxane, carbon black and a curing agent on a heat-resistant resin film having a thickness of 5 to 300 xcexcm and a glass transition point of not lower than 200xc2x0 C. Preferred examples of the heat-resistant film used in this reference as the support are a polyimnide film and a polyamide film. Such supports are superior in flexibility to the metal foil described above. However, because heat conductivity is still low, there remains the problem that the heat resistance in the thickness-wise direction of the sheet remarkably increases.
If the thickness of the metal foil or the plastic film as the support is decreased to reduce the heat resistance in the thickness-wise direction of the heat conductive sheet, wrinkles are likely to occur in the support, or the support is ruptured or elongated, when the sheet is formed. For these reasons, it is difficult to manufacture heat conductive sheet products with high yield, that is, economically, in the practical production process.
Besides the beat conductive sheets described above, International Publication No. WO96/37915 describes an electronic circuit assembly including (a) a heat sink (b) an electronic circuit, and (c) an insulation layer interposed between the heat sink and the electronic circuit, wherein the insulation layer comprises (i) a first heat conductive adhesive layer keeping contact with the heat sink, consisting of an adhesive and heat conductive solid particles, and having a thickness of less than 60 xcexcm, (ii) a heat resistant resin layer having a thickness of less than at most 15 xcexcm and not containing a filler, and (iii) a second heat conductive adhesive layer keeping contact with the electronic circuit, consisting of an adhesive and heat conductive solid particles and having a thickness of less than 60 xcexcm. However, the insulation layer having the three layered structure and used hereby as the heat radiation sheet has a complicated structure, is not easy to produce, and cannot yet solve the problems with the prior art described above.
It is therefore an object of the present invention to provide a heat conductive sheet that solves a large number of problems of the prior art described above, has flexibility, can follow a specific shape such as ruggedness and curvature, which can therefore insure high adhesion and at the same time, high heat conductivity of at least 2.0 W/mK and thus sufficiently reduced heat resistance in the packages, is free from the occurrence of wrinkles and rupture even when its thickness is decreased, and is excellent in formability during the formation of the sheet and in the working factor of bonding. Another object of the present invention is to provide a method of producing such heat conductive sheet.
To accomplish the object described above, the present invention provides a heat conductive sheet including a substrate and a heat conductive resin layer applied to at least one surface of the substrate, wherein the heat conductive resin layer contains a binder resin and a heat conductive filler dispersed in the binder resin.
Further, according to the present invention, there is provided a method of producing a heat conductive sheet including a substrate and a heat conductive resin layer applied to at least one surface of said substrate, comprising the steps of:
supporting said substrate by a support;
applying a film-forming resin composition containing a binder resin and a heat conductive filler to a non-supporting surface of said substrate to form a heat conductive resin layer, and
separating the resulting heat conductive sheet from said support